Please help on this issue, thanks in advance!

A) What I am looking for:

change column width in tree view of a wizard in Odoo 13, rather than default width setting because one column in my wizard is quite wide

B) What I saw:

1) by default, the column of product name is quite wide

2) I tried the solution " The workaround I found " on https://www.odoo.com/es_ES/forum/ayuda-1/changing-column-widths-tree-view-35981  (recommended by https : //www.odoo.com/fr_FR/forum/aide-1/how-to-change-column-width-in-tree-view-one2many-many2many-26693 ), but it still does not work.
btw, I am sure that the css file was loaded well because I defined another css class in the same file and it worked.

C) What I did (reproducing steps):

 1) the tree view (unable to show <>, use # instead):
 #tree editable = "bottom" create = "false" delete = "false" class = "o_my_class" #
                                #field name = "display_name" readonly = " 1 "/ #
                                #field name =" qty_1 "readonly =" 1 "/ #
                            # / tree #

 2) css definition: 
.o_my_class [data-id =" display_name "] {width: 10px; color: red; }